The present invention is concerned with shipboard compass errors specifically with respect to correcting for deviation. Deviation is the error in the compass induced by ferrous objects or electro-magnetic or magnetic fields near the compass. It is distinguished from the departure from a true heading known as variation, which is the difference between geographic north and magnetic north (in the Northern Hemisphere) and the difference between geographic south and magnetic south (in the Southern Hemisphere).
Compass deviation is the result of magnetic interference in the vicinity of the compass, which can be caused for example by a steel centerboard or keel, the engine if not too far away, with its large mass which might be partially magnetized, and by electric motors, radio loud speakers or electric wires carrying current in close proximity to a compass.
In order to adjust a compass for deviation, the majority of small boat compasses have compensators built into the compass assembly. These are small magnets that can be rotated by means of shafts or gear segments to set up magnetic fields opposing local magnetic disturbances, thus cancelling the original cause of deviation. There are situations, however, which make total compensation for deviation extremely difficult if not impossible; for example, the deviation may be so great that compensators cannot take care of the entire amount. In such cases, the creation of a deviation chart is the only practical approach.
It may also happen that a boat owner has never gone through the laborious and time-consuming procedure of compensating the vessel's compass. He may assume that such compensation has occurred previously if he bought his boat from a previous owner, or he may simply have avoided or delayed taking this step. Under these circumstances, the boat owner may wish to take a trip of some distance and have the assurance his compass readings are accurate, or if not accurate, know the deviation quickly and easily. It is in the latter circumstance that the device of the present invention becomes extremely valuable.
Variation error can be eliminated by plotting a course according to magnetic headings; however, on a long trip, especially where bad weather may occur and where visibility becomes poor, it is extremely important to know the deviation for different compass readings.
The apparatus and method of the present invention is directed toward the creation of an accurate deviation chart in the circumstances described above.